A New Method to Control Light in Densely Packed Nanowaveguides

Researchers at the University of California Berkeley and the Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new method to effectively control light pulses in densely packed nanoscale waveguides, paving the way to ultra-compact, ultra-high density integrated photonic circuitry.

Effective control of light pulses is critical to realize high-performance chip-scale quantum computing and optical communications. Xiang Zhang, director of Materials Sciences Division of Berkeley Lab, headed the research wherein optical nanowaveguides were subjected to a mathematical scheme known as adiabatic elimination.

Optical nanowaveguides are photonic models of electronic circuits. Using a combination of adiabatic elimination and coupled systems, which is a typical method used for regulating light movements through a couple of waveguides, the researchers successfully removed the “crosstalk” issue associated with closely packed nanowaveguides.

Zhang is a member of the Kavli Energy NanoSciences Institute at Berkeley (Kavli ENSI) and holds the Ernest S. Kuh Endowed Chair at UC Berkeley. He is also the corresponding writer of the study titled “Adiabatic elimination based coupling control in densely packed subwavelength waveguides” and published in Nature Communications. Lead authors of the study are Haim Suchowski, Michael Mrejen, and Taiki Hatakeyama, while Liang Feng, Chih-hui Wu, Yuan Wang and Kevin O’Brien are the additional authors.

Issues like power consumption and heat dissipation are making integrated electronic circuitry to reach its saturation limits. Photonics is regarded as a better alternative solution as it can transfer more amounts of information at faster speeds and at the same time uses much less power and generates less amount of heat.

In Photonics, electrical signals travel through cables and copper wires, and are substituted by light pulses that transfer data across optical fibers. Nevertheless, the crosstalk issue in coupled system of optical nanowaveguides has represented a major barrier.

"When nanowaveguides in close proximity are coupled, the light in one waveguide impacts the other. This coupling becomes particularly severe when the separation is below the diffraction limit, placing a restriction on how close together the waveguides can be placed. We have experimentally demonstrated an adiabatic elimination scheme that effectively cuts off the cross-talk between them, enabling on-demand dynamical control of the coupling between two closely packed waveguides. Our approach offers an attractive route for the control of optical information in integrated nanophotonics, and provides a new way to design densely packed, power-efficient nanoscale photonic components, such as compact modulators, ultrafast optical signal routers and interconnects," said Zhang.

"A general approach to achieving active control in coupled waveguide systems is to exploit optical nonlinearities enabled by a strong control pulse. However this approach suffers from the nonlinear absorption induced by the intense control pulse as the signal and its control propagate in the same waveguide," Zhang added.

The researchers applied the adiabatic elimination concept which has so far shown to be effective in atomic physics and similar other research areas. The aim behind the adiabatic elimination concept is to break down huge dynamic solutions into smaller units by means of fast vs. slow dynamics. This concept was applied to the coupled optical nanowaveguides by integrating a third waveguide in the center of the combined waveguides.

Each of the three waveguides is separated by just 200nm. Such proximity would lead to an inherent cross-talk issue and prevent control of the coupled system. However, the center waveguide works in a dark mode and since it does not build up any light, it does not appear to take part in the light exchanging process between the two external waveguides.

"Picture three buckets side-by-side with the first being filled with water from a tap, the middle being fed from the first bucket though a hole while feeding the third bucket through another hole. If the flow rate into the middle bucket is equal to the flow rate out of it, the second bucket will not accumulate water. This, in a basic manner, is adiabatic elimination. The middle bucket allows for some indirect control on the dynamics compared to the case in which water goes directly from the first bucket to the third bucket," said co-lead author Mrejen.

"Even though the dark waveguide in the middle doesn’t seem to be involved, it nonetheless influences the dynamics of the coupled system. By judiciously selecting the relative geometries of the outer and intermediate waveguides, we achieve adiabatic elimination, which in turn enables us to control the movement of light through densely packed nanowaveguides. Until now, this has been almost impossible to do," said co-lead author Suchowski, who is now with Tel Aviv University.